Answer

$ 42n^{2}$

Work Step by Step

$ \sqrt {1764} \times \sqrt {n^{4}} =\sqrt {1764} \times n^{2}$
In order to see if a radical is in simplified form, see if any of its factors are perfect squares (meaning that their square root will be an integer). We see that 1764 is a perfect square, so we know that we can simplify:
$\sqrt {1764} \times n^{2}= 42n^{2}$